Surface emitting laser structures such as vertical cavity surface emitting lasers (VCSELs) have gained significant importance in the field of optical communications. The high switching speed offered by semiconductor lasers employing, for example, III-V alloy compounds have made such devices a logical choice for optical transmitters. For several reasons including; reliability, ease of coupling, and testing, VCSELs have gained acceptance over the more conventional edge emitting devices. VCSELs are typically fabricated using well known planar processes and equipment and are well suited for integration with other active and passive components.
Typically, VCSELs have a common back contact and an apertured contact on the emitting face with the emission from the optical device exiting through the aperture. The contact aperture is usually circular as this is better suited for alignment with optical fibers.
Polarization of the light from such standard VCSELs is unpredictable as it tends to be randomly oriented from one device to another. Further, polarization may switch in operation particularly at high speeds. The polarization of light emitting from a VCSEL can be important especially when used in conjunction with polarization sensitive components and efforts have been made in an attempt to tailor or control VCSEL polarization.
In an article published by Fiedler et al. entitled “High Frequency Behaviour of Oxidized Single-Mode Single Polarization VCSELs with Elliptical Current Aperture”, Lasers and Electro-Optic Society annual meeting 1996 IEEE volume 1, 1996, pages 211 to 212 there is discussed a technique wherein oxidized VCSELs are provided with elliptical current apertures in an effort to control polarized single mode light emission.
An article entitled “Impact of In-Plane Anistropic Strain on the Polarization Behavior of Vertical-Cavity Surface-Emitting Lasers” by Panajotov et al. (Applied Physics Letters, Volume 77, Number 11, Sep. 11, 2000) discloses an externally induced in-plane anisotropic strain applied to a VCSEL in order to demonstrate the presence of switching between two fundamental modes with orthogonal linear polarization.
Externally applied strain or stress to control polarization of VCSELs was also described in U.S. Pat. No. 6,188,711 to Corzine et al.
U.S. Pat. No. 6,002,705 which issued Dec. 14, 1999 to Thornton describes wave length and polarization multiplexed vertical cavity surface emitting lasers in which stress inducing elements are disposed on a free surface of the laser device. The stress inducing elements are made of a material having a higher coefficient of thermal expansion than the material which comprises the surface layer of the laser device.
U.S. Pat. No. 5,953,962 which issued Sep. 14, 1999 to Pamulapati et al. describes a strain induced method of controlling polarization states in VCSELs. In the 5,953,962 patent the VCSEL is eutectically bonded to a host substrate which has a predetermined anisotropic coefficient of thermal expansion. During the forming process a uniaxial strain is induced within the laser cavity.
U.S. Pat. No. 6,154,479 which issued Nov. 28, 2000 to Yoshikawa et al. discloses a VCSEL in which control of the polarization direction is effected by limiting the cross sectional dimension of the top mirror so as to limit only a single fundamental transverse mode in the wave guide provided by the mirror. A non-circular or elliptical device is created so as to control the polarization.
U.S. Pat. No. 5,995,531 which issued Nov. 30, 1999 to Gaw et al. also discloses an elliptical cross sectional top mirror which is formed into a ridge with the ridge being etched down into an ion implantation region to form an elongated shape so as polarize light emitted by the device. It is also known in the prior art to use rectangular air-post structures, asymmetric oxide apertures and an elliptical hole on the bottom emitting laser as ways of controlling polarization.
All of the above methods involve complex fabrication and/or processing steps and what is needed is a simple technique of controlling and stabilizing polarization of VCSELs.
One solution to the aforementioned problem of polarization switching particularly when the VCSEL is operated with large modulating signals is described in Applicant's co-pending British Application 1006192.6 filed Jul. 3, 2001.
Typically, laser action in a VCSEL is in one longitudinal mode only due to the short cavity length. On the other hand, laser action in a VCSEL may be supported in multiple transverse modes if the emitting aperture is large enough to support such operation. It is known that in multi mode VCSELs mode partition noise (MPN) occurs when individual modes compete for carriers. With a single mode there is no MPN to degrade the performance of the VCSEL. Further, if the single mode is the fundamental mode the beam will have a Gaussiam electromagnetic field distribution. The Gaussiam beam has a smaller spot size and a smaller divergence than the higher order lateral modes in the VCSEL. This is an advantage for all applications wherein a small spot and low divergence is important. This includes applications wherein a small circular spot is an advantage i.e. read-out technology. The small spot size is of great benefit because the alignment to the fiber becomes easier. In addition, a small spot size and a low divergence angle makes it possible to launch the light into the core of a fiber which in turn increases the distance data can be sent at a high rate.
Polarization switching is a known phenomenon, as previously discussed, and such polarization switching may cause mode hopping in VCSELs. Thus, a switching (or a hopping) between the polarization modes may alter the static and dynamic properties of the laser and significantly degrade the communication link. Additionally, transmission could be degraded further because of an isotropy in the optical fiber. Even though the circular output profile of the single mode is of great benefit for the coupling to a fiber, the unstable polarization state caused by the symmetry of the aperture will tend to reduce the benefit. Thus, by introducing asymmetry the polarization can be stabilized or at least controlled with a relatively small trade off.